docs: restructure introduction (#1151)

Brought the documentation more in line with the other documentation
elements.

- Shortened the introduction page
- Introduced the same PlanningAI Concepts page as on other sites (minus
the reference to the platform). This was originally also in the
"introduction"
- Introduced an FAQ to place information which people will look for
(like the licensing). Good place to add additional things Sales get
asked about often.

Also removed from the introduction page:

- The list of different Use Cases. While that is somewhat valuable, i'd
like to re-introduce those in the form of images. I left the current
image which already shows 4 use-cases. Longer term, let's introduce an
"Example Use Cases" heading to the documentation where we make these
shine a bit more.
- Note on Backwards Compatibility. I think this does not belong in an
Intro. I have removed it for now as I think the expectation is always
that this information should flow through the Changelog.
- How to use Timefold Solver => Should be part of the quickstarts, so
pushing people to the quickstarts sooner.

Author: TomCools

docs/src/modules/ROOT/nav.adoc

@@ -1,10 +1,11 @@
-* xref:introduction/introduction.adoc[leveloffset=+1]
-* Quickstart
+* xref:introduction.adoc[leveloffset=+1]
+* Getting Started
 ** xref:quickstart/overview.adoc[leveloffset=+1]
 ** xref:quickstart/hello-world/hello-world-quickstart.adoc[leveloffset=+1]
 ** xref:quickstart/quarkus/quarkus-quickstart.adoc[leveloffset=+1]
 ** xref:quickstart/spring-boot/spring-boot-quickstart.adoc[leveloffset=+1]
 ** xref:quickstart/quarkus-vehicle-routing/quarkus-vehicle-routing-quickstart.adoc[leveloffset=+1]
+* xref:planning-ai-concepts.adoc[leveloffset=+1]
 * Using Timefold Solver
 ** xref:using-timefold-solver/overview.adoc[leveloffset=+1]
 ** xref:using-timefold-solver/configuration.adoc[leveloffset=+1]
@@ -32,4 +33,5 @@
 ** xref:upgrading-timefold-solver/overview.adoc[leveloffset=+1]
 ** xref:upgrading-timefold-solver/upgrade-to-latest-version.adoc[leveloffset=+1]
 ** xref:upgrading-timefold-solver/upgrade-from-optaplanner.adoc[leveloffset=+1]
+** xref:upgrading-timefold-solver/backwards-compatibility.adoc[leveloffset=+1]
 * xref:enterprise-edition/enterprise-edition.adoc[leveloffset=+1]

docs/src/modules/ROOT/pages/.index.adoc

@@ -18,13 +18,15 @@ ifndef::backend-pdf[]
 image::shared/timefold-logo.png[align="center"]
 endif::[]
 
-include::introduction/introduction.adoc[leveloffset=+1]
+include::introduction.adoc[leveloffset=+1]
 include::quickstart/.quickstart.adoc[leveloffset=+1]
+include::planning-ai-concepts.adoc[leveloffset=+1]
 include::using-timefold-solver/.using-timefold-solver.adoc[leveloffset=+1]
 include::constraints-and-score/.constraints-and-score.adoc[leveloffset=+1]
 include::optimization-algorithms/.optimization-algorithms.adoc[leveloffset=+1]
 include::responding-to-change/responding-to-change.adoc[leveloffset=+1]
 include::integration/integration.adoc[leveloffset=+1]
 include::design-patterns/design-patterns.adoc[leveloffset=+1]
+include::frequently-asked-questions.adoc[leveloffset=+1]
 include::upgrading-timefold-solver/.upgrading-timefold-solver.adoc[leveloffset=+1]
 include::enterprise-edition/enterprise-edition.adoc[leveloffset=+1]

docs/src/modules/ROOT/pages/frequently-asked-questions.adoc

@@ -0,0 +1,15 @@
+= FAQ
+:doctype: book
+:icons: font
+
+== How is Timefold Solver Licensed?
+
+Timefold Solver Community Edition is _open source_ software,
+released under http://www.apache.org/licenses/LICENSE-2.0.html[the Apache License 2.0].
+This license is very liberal and allows reuse for commercial purposes.
+Read http://www.apache.org/foundation/licence-FAQ.html#WhatDoesItMEAN[the layman's explanation].
+
+Timefold Solver Enterprise Edition is a commercial product
+that offers xref:enterprise-edition/enterprise-edition.adoc#enterpriseEditionFeatures[additional features]
+to scale out to very large datasets.
+To find out more, see xref:enterprise-edition/enterprise-edition.adoc[Enterprise Edition section] of this documentation.

docs/src/modules/ROOT/pages/introduction.adoc

@@ -0,0 +1,30 @@
+[#introduction]
+:page-aliases: ../index.adoc
+:doctype: book
+:sectnums:
+:icons: font
+
+[#whatIsTimefold]
+= What is Timefold Solver?
+
+Every organization faces planning problems: providing products or services with a limited set of _constrained_ resources (employees, assets, time, and money).
+Timefold Solver’s xref:planning-ai-concepts.adoc[PlanningAI] optimizes these problems to do more business with fewer resources using Constraint Satisfaction Programming.
+
+> Want to dive right into it? Follow our xref:quickstart/overview.adoc[Quickstart Example] to tackle your first planning problem in minutes.
+
+https://timefold.ai[Timefold Solver] is a lightweight, embeddable constraint satisfaction engine which optimizes planning problems.
+Example usecases include:
+
+.Timefold Solver's use cases include vehicle routing, employee scheduling, rostering, bin packing, and equipment scheduling.
+image::introduction/useCaseOverview.png[align="center"]
+
+Timefold Solver is 100% pure Java^TM^ and runs on Java {java-version} or higher.
+It xref:integration/integration.adoc#integration[integrates very easily] with other Java^TM^, Python, and other technologies.
+Timefold Solver works on any Java Virtual Machine and is compatible with the major JVM languages and all major platforms.
+It also supports Kotlin and Python.
+
+:!sectnums:
+== Next
+
+* Follow the xref:quickstart/overview.adoc[Quickstart Example] to tackle your first planning problem.
+* Learn about some important concepts used in the realm of xref:planning-ai-concepts.adoc[PlanningAI].